[slloyd]

the internal resistance is affected by discharge rate because higher current equals higher temperature which changes the resistance.

are you trying to determine your state of charge during normal robot operation? maybe there is another way that would be easier. using the onboard A-to-D conversion and scaling the software to monitor the battery between 10V and 15V. i think there was an official FIRST post about using external circuit to do this.. i forget now, was a year ago when i saw this.

Keep in mind that your battery is considered DEAD when it reaches 12V and needs to be recharged. if you allow your battery to drain lower, like 11V, you will cause damage. you may notice that the next time you charge your battery it does not reach the same voltage off charge, or that it doesn't last as long when applied to the exact same load.

also, during a match when you are pulling heavy current you will reach 12V. then when you disconnect your battery and measure the voltage it may say 12.18V or something... you might think that its OK to put back on the robot, but this is misleading. under loading conditions your battery may drop below 12V and this is not good for your battery and needs to be recharged.

same sort of thing applies to your backup battery, though being nicad they are more tolerant. 7.2V is the target here.

slloyd